Tuning Fe-spin state of FeN4 structure by axial bonds as efficient catalyst in Li-S batteries

In lithium-sulfur (Li-S) batteries, the root-cause solution is to accelerate the polysulfide conversion to their end products by electrocatalysts. Herein, we put forward a molecular spin engineering to break the symmetry of Fe-N-C and achieve the spin modulation for triplet-to-singlet conversion by the ligand field theory. The Fe-N-C shows an 3d-electronic structure of (dxy)2(dxz)2(dyz)2 and dramatically speeds up the LPSs conversion kinetics. According to the density functional theory (DFT), there is an upshift of energy levels after the adsorption of LPSs on Fe center. As a result, the optimized catalyst delivers a capacity fading rate of 0.026% per cycle at 2 C. In addition, a high capacity of 1042 mA h g−1 is achieved with satisfied capacity retention with the sulfur loading of about 4 mg cm−2. This strategy provides a novel route for the regulation of Fe-spin state and the exploration of catalytic effect in Li-S batteries.